Harley is an immortal cryptid with a curse that keeps them immortal through continuous reincarnation from their cursed mask.
Back in medieval times, Harley was a harlequin when they were alive. After they died, their body was used as an experiment by a curious mortician ahead of their times, hellbent on the promise of conceiving the key to immortality for reasons unknown. This was done by making a clay mask made out of Harley's ashes that can take the soul out of the wearer's body and put Harley in their body instead via putting on the mask.
Harley keeps continually getting revived over and over due to really dumb and curious people who dare to put on the mask, despite Harley trying their best efforts to hide the mask as best as possible.
Harley can't die by their own hands, but can by someone else's, however it takes a couple blows to do so. Because of this, they live out their days just trying to cope with this unfortunate curse, along with the centuries long guilt of snatching innocent people's lives away from them.